Shaft and tunnel excavation in hard rock involves three distinct operations, namely drilling and blasting; mucking (i.e. removing the broken rock); and then lining the shaft or tunnel (stabilizing it with rock bolts, screen, steel or concrete), and equipping the shaft or tunnel. To improve the daily rate of excavation, the mucking, lining and equipping operations may be optimized largely by upscaling the equipment used. However the drilling and blasting operations are much less easily amenable to being optimized, and the method of drilling and blasting, and the skill of the driller, usually determine the daily advance rate. The present invention is concerned with improving the drilling and blasting operation by providing an improved method for breaking a full face of rock and an apparatus for carrying out the method.
The full face method of mine shaft excavation is well known in the mining industry. This method involves the breakage and removal of rock or earth over the "full face" or full intended diameter of the shaft. This may be contrasted with the benching method of shaft excavation which involves breaking and excavating smaller portions of the "face" of the shaft by creating a series of benches or steps.
The full face method has been successfully used in many countries in the world and particularly South Africa, where in the past, lower labour costs and large drilling and bottom cleaning crews have resulted in high speed shaft sinking rates. The South African method employed a V-cut for excavating the full face. Blast holes in the V-cuts were drilled by a mechanized shaft drilling machine. One difficulty with this method of shaft construction was that because blasting a V-cut throws rock a long distance, the sinking stage had to be moved a large distance (at least 200 feet) from the shaft bottom before blasting. This was time consuming and inefficient. In addition, the process of equipping the shaft could not be carried on simultaneously with excavation. This led to a less efficient rate of shaft completion. Further, the V-cut limited the length of round (i.e. the depth for each blast cycle) that could be blasted.
An alternative method for breaking a full face of rock is the "shatter" or "burn" cut. This method offers the advantage over the V-cut method of only having to raise the sinking stage a short distance (approximately 30 feet) from the shaft bottom in shaft sinking. When tunnelling, the broken rock is not thrown as far along the tunnel, reducing cleaning time. However a major disadvantage of the burn cut (as will be described further below) is the high degree of skill required to break the cut to the full extent drilled.
More specifically, the shatter cut method involves drilling a number of holes into the rock. Nearly all the holes are filled with explosives and act as blast holes. A small number of holes are left uncharged to act as relief holes into which the fragmented rock may expand. During blasting, provided that the holes have been properly drilled and charged, the compression or strain waves travelling through the rock fragment or shatter the rock (hence the name), without throwing the rock long distances.
Many different patterns of relief holes and blast holes have been used in association with the shatter cut method. It is well recognized however that, regardless of the pattern chosen, proper spacing and alignment of the blast holes relative to the relief holes is necessary to successfully break the rock to the entire desired number of "bootlegs" may exist (bootlegs are the remnants of blast holes at the base of the round) as well as brittle, partially fragmented rock. The existence of such bootlegs and such brittle rock can present a major hazard to workers and may seriously delay the excavation of the next stage of the shaft, since they must all be marked, and no drilling can occur closer than 6 inches to them. However they can be difficult to find, since they may be hidden under broken rock and/or water.
It has been found that the shatter cut method achieves its best results in rock fragmentation when the blast holes are drilled exactly parallel to the free face (inside surface) of the relief holes. Parallel holes ensure that the cylindrically expanding compressive strain wave radiating from the detonated blast hole is reflected back 180.degree. upon meeting the free face of the relief hole. This reflection increases the occurrence of reflection breakage or "spalling" and allows for greater fragmentation of the rock, reducing the likelihood of bootlegs and brittle rock fragments.
It has further been found that there is a critical radial distance between the blast hole and the relief hole beyond which effective fragmentation is less likely to occur. The "radial cracking" caused by the compressive strain wave radiating from the blast hole, and the "spalling" caused by the reflected strain wave radiating from the relief hole, together influence rock fragmentation. As the radial distance between the blast hole and the relief hole increases, the influences of "radial cracking" and "spalling" upon rock fragmentation decrease. Beyond the critical radial distance, successful breakage of the round is jeopardized. As a consequence, blast holes are typically drilled slightly within this critical radial distance from the relief hole.
Great care must be taken to ensure that the above factors are met. A non-parallel blast hole may decrease the occurrence of "spalling" and also may stray beyond the critical radial distance from the relief hole at some point along the depth of the round and affect the success of rock fragmentation. However, achieving such parallelism, particularly for long holes and in hard rock, is extremely difficult and a highly skilled driller is required. Moreover, since the characteristics of rock in one geographic area may differ greatly from those of rock in another geographic area, a driller experienced at one locality may have difficulty drilling long parallel holes at another locality.
One way to improve rock fragmentation is to drill larger diameter relief holes. The greater area of the free face of the larger relief holes increases the influences of "radial cracking" and "spalling" upon the rock. However relief holes having a diameter greater than 150 mm (6 inches) have not normally been used in practice because of the difficulties in drilling these holes. Because the equipment used to drill large relief holes has been so large, therefore on the few occasions on which large relief holes have been drilled, it has been necessary first to drill the relief hole and then afterwards to drill the individual blast holes around the relief hole. The two stage drilling process has been so time consuming that the benefits associated with drilling large diameter relief holes have been lost. Shaft sinking and tunnel excavation in hard rock have therefore for many years been a relatively slow and costly process.